1. Field of Invention
This invention relates to an electronic circuit, a semiconductor device, electronic equipment, and a timepiece.
2. Description of Related Art
An electronic circuit that is known in the art comprises a constant-voltage generation circuit that outputs a constant voltage and a crystal oscillation circuit that is driven by this constant voltage. This kind of electronic circuit is widely used in applications such as timepieces, telephones, and computer terminals.
Recent trends in the miniaturization of electronic equipment have made it particularly common to fabricate such electronic circuitry as compact, low-power ICs.
However, electronic circuitry formed as an IC has a problem in that a constant voltage that is output from the constant-voltage generation circuit varies with the effects of temperature.
This is particularly important with a crystal oscillation circuit that is driven by a constant voltage output by a constant-voltage generation circuit because, if that constant voltage should change, the oscillation frequency of the crystal oscillation circuit will also change. This causes a problem in an electronic circuit that generates a reference clock signal for operation based on the oscillation frequency of this crystal oscillation circuit, in that accurate time-keeping is not possible. If a wristwatch is taken by way of example, the environment in which such a wristwatch is used can range from low temperatures to high temperatures. If prior-art electronic circuitry is used in such a wristwatch, therefore, variations in the constant voltage that is output from the constant-voltage generation circuit can cause errors in the time displayed thereby.
It is necessary to set the absolute value of the constant voltage that is output from the constant-voltage generation circuit to be always greater than the absolute value of the oscillation-stopped voltage of the crystal oscillation circuit. If this voltage falls below the oscillation-stopped voltage, the crystal oscillation circuit will no longer be able to operate.
It is known that the power consumption of the crystal oscillation circuit is proportional to the square of the constant voltage supplied from the constant-voltage generation circuit. To reduce the power consumption of the electronic circuitry, therefore, it is necessary to set the value of the constant voltage that is output from the constant-voltage generation circuit to be as small as possible, within a range that satisfies the condition that it is greater than the oscillation-stopped voltage of that crystal oscillation circuit.
If such electronic circuitry is formed as a semiconductor integrated circuit, the effects of factors such as errors in impurity implantation will cause subtle changes in the value of the constant voltage output from the constant-voltage generation circuit and the value of the oscillation-stopped voltage of the crystal oscillation circuit.
Since it is not possible to finely adjust the value of the constant voltage that is output from the constant-voltage generation circuit in prior-art electronic circuitry, it is necessary to set the value of this constant voltage to have a sufficiently large margin over the expected value of the oscillation-stopped voltage, from consideration of the risk of a large variation therein. This means that the crystal oscillation circuit is driven by a voltage that is larger than necessary, raising a problem in that it is difficult to reduce the power consumption of the electronic circuitry from this aspect too.
The present invention is devised in the light of the above problems and has as a first objective thereof the provision of an electronic circuit, semiconductor device, electronic equipment, and timepiece wherein the value of the constant voltage that is output from the constant-voltage generation circuit is not affected greatly by changes in temperature.
Another objective of this invention is to provide an electronic circuit, semiconductor device, electronic equipment, and timepiece wherein the value of the constant voltage that is output from the constant-voltage generation circuit can be adjusted finely.
In order to achieve the first of the above objectives, there is provided an electronic circuit having a constant-voltage generation circuit for creating a constant voltage, according to a first aspect of this invention. This constant-voltage generation circuit comprises a first voltage creation circuit for creating a reference voltage, and a second voltage creation circuit for creating the constant voltage to have a predetermined relationship with the reference voltage. The first voltage creation circuit comprises a first constant-current source for supplying a constant current, and a circuit having a first voltage-control transistor through which the constant current is passed and which outputs the reference voltage with reference to a predetermined potential. The constant current is set to a value within a saturated operating region of the first voltage-control transistor.
The second voltage creation circuit may comprise a differential amplifier for amplifying the difference between the reference voltage and a comparison voltage, a second constant-current source for supplying a constant current, a circuit having a second voltage-control transistor to which the constant current is supplied, and an output transistor which is connected in series with the circuit having the second voltage-control transistor to be supplied with the constant current, the resistance of the output transistor being controlled by an output of the differential amplifier; wherein the comparison voltage is output from one end of the circuit having the second voltage-control transistor, using a predetermined potential as reference, while the constant voltage being output from another end of the circuit having the second voltage-control transistor; and wherein the constant current is set to a value within a saturated operating region of the second voltage-control transistor.
According to a second aspect of this invention, there is provided an electronic circuit having a constant-voltage generation circuit for creating a constant voltage. This constant-voltage generation circuit comprises a first voltage creation circuit for creating a reference voltage, and a second voltage creation circuit for creating the constant voltage and a comparison voltage having a predetermined relationship with the constant voltage. The second voltage creation circuit comprises: a differential amplifier for amplifying the difference between the reference voltage and the comparison voltage; a second constant-current source for supplying a constant current; a circuit having a second voltage-control transistor to which the constant current is supplied; and an output transistor which is connected in series with the circuit having the second voltage-control transistor to be supplied with the constant current, the resistance of the output transistor being controlled by an output of the differential amplifier. The comparison voltage is output from one end of the circuit having the second voltage-control transistor, using a predetermined potential as reference, while the constant voltage being output from another end of the circuit having the second voltage-control transistor. The constant current is set to a value within a saturated operating region of the second voltage-control transistor.
This aspect of invention makes it possible to reduce variations in the voltage between the ends of the voltage-control transistor to an ignorable level, even if the value of the constant current supplied from the constant-current source varies slightly because of temperature changes in the environment in which the electronic circuit is used, by setting the value of the constant current supplied by the constant-current source to be within the saturated operating region of the voltage-control transistor. Therefore, the value of at least one of the reference voltage and the comparison voltage output from at least one of the first voltage creation circuit and the second voltage creation circuit remains substantially constant, regardless of the effects of temperature changes, so that the constant-voltage generation circuit can always output a constant voltage.
In this manner, the electronic circuit ensures that the constant-voltage generation circuit thereof can generate and output a constant voltage that does not vary greatly, even if the ambient temperature changes.
In particular, it is possible to maintain a constant oscillation frequency output from a crystal oscillation circuit, even if the ambient temperature varies, by using the constant voltage that is output from this constant-voltage generation circuit as a voltage for driving the oscillation circuit. As a result, it is possible to create an accurate operating reference signal from the oscillation output of this crystal oscillation circuit.
It is preferable to use a field-effect transistor as the voltage-control transistor. It is more preferable to use a field-effect transistor wherein the gate and drain thereof have been short-circuited.
In order to achieve the other of the above described objectives, there is provided an electronic circuit having a constant-voltage generation circuit for creating a constant voltage, according to a third aspect of this invention. This constant-voltage generation circuit comprises a first voltage creation circuit for creating a reference voltage, and a second voltage creation circuit for creating the constant voltage to have a predetermined relationship with the reference voltage. The first voltage creation circuit comprises a first constant-current source for supplying a constant current, and a circuit having a first voltage-control transistor through which the constant current is passed and which outputs the reference voltage with reference to a predetermined potential. As the first voltage-control transistor, one transistor is selected from a plurality of transistors having mutually different current amplification ratios.
The second voltage creation circuit may comprise a differential amplifier for amplifying the difference between the reference voltage and a comparison voltage, a second constant-current source for supplying a constant current, a circuit having a second voltage-control transistor to which the constant current is supplied, and an output transistor which is connected in series with the circuit having the second voltage-control transistor to be supplied with the constant current, the resistance of the output transistor being controlled by an output of the differential amplifier; wherein the comparison voltage and the constant voltage are output with reference to a predetermined potential from one end and another end of the circuit having the second voltage-control transistor; and wherein one transistor from a plurality of transistors having mutually different current amplification ratios is selected as the second voltage-control transistor.
According to a fourth aspect of this invention, there is provided an electronic circuit having a constant-voltage generation circuit for creating a constant voltage. This constant-voltage generation circuit comprises a first voltage creation circuit for creating a reference voltage, and a second voltage creation circuit for creating the constant voltage and a comparison voltage having a predetermined relationship with the constant voltage. The second voltage creation circuit comprises: a differential amplifier for amplifying the difference between the reference voltage and the comparison voltage; a second constant-current source for supplying a constant current; a circuit having a second voltage-control transistor to which the constant current is supplied; and an output transistor which is connected in series with the circuit having the second voltage-control transistor to be supplied with the constant current, the resistance of the output transistor being controlled by an output of the differential amplifier. The comparison voltage and the constant voltage are output with reference to a predetermined potential from one end and another end of the circuit having the second voltage-control transistor: As the second voltage-control transistor, one transistor is selected from a plurality of transistors having mutually different current amplification ratios.
In an electronic circuit in accordance with this aspect of the invention, any desired transistor can be selected from a plurality of transistors having different current amplification ratios, for use as the voltage-control transistor. This makes it possible to finely adjust the value of at least one of the reference voltage and the comparison voltage, so that the value of the constant voltage that is output from the voltage creation circuit can be finely adjusted.
By using the constant voltage that is output from the constant-voltage generation circuit as a voltage for driving a crystal oscillation circuit, it is possible to adjust this drive voltage finely to the necessary minimum limit to match the oscillation-stopped voltage of the crystal oscillation circuit. This means that it is possible to drive the electronic circuitry, particularly the crystal oscillation circuit, stably at a low power consumption.
In particular, it is possible to form circuitry that outputs the optimal constant voltage with respect to the oscillation-stopped voltage of the crystal oscillation circuit, during the fabrication of the electronic circuit. Use of this configuration makes it possible to finely adjust the value of the constant voltage that is output from the constant-voltage generation circuit in such a manner that it is greater than the oscillation-stopped voltage and is also at the necessary minimum value, even if slight variations occur in the characteristics of the constant-voltage generation circuit or the oscillation-stopped voltage of the crystal oscillation circuit, during the process of fabricating the semiconductor device. Since this fine adjustment can be done during the fabrication of the electronic circuit, or more specifically during the fabrication of the semiconductor device, it is thus possible to fabricate a semiconductor device that is provided with an electronic circuit in which a crystal oscillation circuit can be driven stably and which also has a low power consumption, with a good yield.
Furthermore, it is preferable to use a field-effect transistor as each transistor. It is more preferable to use a field-effect transistor wherein the gate and drain thereof have been short-circuited.
An electronic circuit according to a fifth aspect of this invention comprises a constant-voltage generation circuit for outputting a predetermined constant voltage, and a crystal oscillation circuit that is driven to oscillate by the constant voltage supplied from the constant-voltage generation circuit. The temperature characteristics of the oscillation-stopped voltage of this crystal oscillation circuit and the constant voltage that is output from this constant-voltage generation circuit are set to be substantially the same.
It is therefore possible to implement an electronic circuit that can drive a crystal oscillation circuit stably and at an even lower power consumption, by using the constant voltage that is output from the constant-voltage generation circuit to drive the crystal oscillation circuit.
The constant-voltage generation circuit may comprise at least one voltage-control transistor supplied with a predetermined constant current, for outputting at least one of the reference voltage and the comparison voltage for controlling the constant voltage to be output; and the constant current may be set to a value such that the total magnitude of voltage variation within a guaranteed operating temperature range of the voltage-control transistor is substantially the same as the magnitude of variation of the oscillation-stopped voltage within the guaranteed operating temperature range.
With this configuration, the value of the constant voltage that is output from the constant-voltage generation circuit can be set to a slightly higher value than the oscillation-stopped voltage of the crystal oscillation circuit, within the entire temperature range required as the operating environment of the crystal oscillation circuit. As a result, the crystal oscillation circuit can be driven for a long time both stably and with a low power consumption, whatever the temperature environment it may encounter.
The constant current may be set to a value such that the magnitude of voltage variation within a guaranteed operating temperature range of the first and second voltage-control transistors is one half the magnitude of variation of the oscillation-stopped voltage within the guaranteed operating temperature range.
This ensures that the value of the constant voltage that is output from the constant-voltage generation circuit is set to the minimum voltage that enables the crystal oscillation circuit to operate. Thus the crystal oscillation circuit can be driven for a long time both stably and with a low power consumption.
The absolute value of the constant voltage may be greater than the absolute value of the oscillation-stopped voltage of a crystal oscillation circuit supplied with the constant voltage.
The constant-current source used in the constant-voltage generation circuit is preferably fabricated to supply a constant current having a negative temperature characteristic. This makes it possible to avoid damage to the circuit by a too-large constant current that might otherwise occur when the ambient temperature rises.
A semiconductor device in accordance with this invention comprises the above described electronic circuit.
Electronic equipment in accordance with this invention comprises the above described electronic circuit or semiconductor device, and an operating reference signal is generated from the oscillation output of the crystal oscillation circuit.
A timepiece in accordance with this invention comprises the above described electronic circuit or semiconductor device, and a timepiece reference signal is generated from an oscillation output of the crystal oscillation circuit.